A man found wandering around Paris, unable to speak, is finally identified as the harbor-master of Ouistreham, Capt. Yves Joris. His house-keeper and companion, Julie Legrand, comes to Paris to get him, and M accompanies them back to Ouistreham. Joris had been shot in the head, and excellently operated on, but doesn't seem aware of who he is. Among his letters had been found a bank statement that someone had deposited 300,000 francs to his account. M has hardly passed the night at his hotel when he is called to the Joris house: Capt. Joris has been poisoned, and dies. Julie's brother, Big Louis, is an ex-con seaman who works on the Saint-Michel, which had been in the night the Capt. disappeared. M visits the Saint-Michel, which has docked again, and meets Louis Legrand (Big Louis) and Capt.Yves Lannec. Louis leaves to sleep on a dredger, and M discovers that someone else had been there as well, but no one is talking. M visits the house of the mayor, Ernest Grandmaison, president of a shipping company, who is not helpful, and trails Julie to a shrine in the dunes, where she has wished for "success for her brother's project". But she won't reveal what it is. M sends for Lucas, who follows the man from the dredger to near the mayor's house, where he loses him. In the house they see Big Louis and the mayor, normally unacquainted. M sets the forces of the police to work and has the missing man arrested and brought back to Grandmaison's, where he confronts Louis and the mayor with Jean Martineau, a nationalized Norwegian. All refuse to talk. M learns that the mayor's wife, Hélène Grandmaison, knows Martineau as Raymond. He returns with Grandmaison to his offices in Caen, where Ernest Grandmaison shoots himself, and the story is revealed. The Norwegian was his cousin, Raymond Grandmaison, who'd been in love with Hélène. He'd been caught dipping into the company funds, and Ernest had forced him to leave France forever, after which he himself married Hélène. But she'd been pregnant with Raymond's child. 15 years later Raymond had been trying to steal back his son, and enlisted the help of Louis and Joris, as well as the Saint-Michel. A slip-up resulted in Joris being shot, the plan aborted. Martineau managed to get Joris patched up, but when he returned, the mayor feared he'd reveal the truth if he came to his senses, and poisoned him.